There are two antigenically different types of monoamine oxidase in brain. The first type is antigenically identical to liver mitochondrial MAO. The second type, "brain specific" MAO, does not cross-react with antibody to liver MAO and has been found only in nervous tissue. The two types of MAO are separated immunochemically. The substrate and inhibitor specificity as well as the regional and subcellular distribution of the "brain specific" MAO are being investigated and compared to that of liver mitochondrial MAO. Further research will be directed toward elucidating the function of these two types of MAO in brain. An attempt will also be made to find a MAO inhibitor that selectively inhibits "brain specific" MAO. Such an inhibitor might be a more effective antidepressant with fewer side effects.